When a device determines that a packet does not belong on the local subnet (which would
called off-net), it will send the packet to the router (its default gateway) on its subnet. The
router must then determine where to send the packet.
Other
Networks
Host A
Host B
10.1.3.0/24
172.16.103.0/24
172.16.104.0/24
10.1.2.76
255.255.255.0(/24)
10.1.1.34
255.255.255.0(/24)
10.1.1.37
255.255.255.0(/24) 10.1.1.0/24
10.1.2.0/24
260 Chapter 4: LAN Connections
All routers have routing tables. Depending on the location of the router in the network
hierarchy, the table can be small and simple or large and complex. Figure 4-16 shows a
packet traveling from host A to host B on different networks. As the packet travels between
the adjoining networks, the routers must reference their routing tables to determine where
to send the packet next.
Figure 4-16 Routing Tables
The router populates the routing table with the network-signi?¬?cant part of all known
networks, to compare the destination network addresses of packets that need to be
forwarded. If the network is not directly attached to the router, the router stores the address
of the next-hop router to which the packet should be forwarded. For routers to function
without the need to store all destination networks in their tables, they use a default route to
which packets not matching any entry in the route table are forwarded.
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